An electronic image pickup apparatus, or a so-called digital still camera, employing a CCD (Charge Coupled Device) image sensor or a C-MOS image sensor as a photo-voltaic transducer device, is found in widespread applications.
This digital still camera is routinely provided with an automatic focusing function for automatic focus setting. While there are a large variety of automatic focusing systems for a digital still camera, such a system in which, as a focusing lens is moved, an image is photographed by the CCD at each of different positions of the focusing lens, in order to search a position of the maximum contrast of the photographed image, represents a mainstream. With this system, the production cost is not that high because routine electrical image information acquisition means is used such that there is no necessity of providing a distance sensor dedicated to the automatic focusing.
In effecting such automatic focusing employing the photographed image, luminosity of the photographed image falls short under a low luminosity environment, such as a dark place, and hence the contrast cannot be detected. Thus, supplementary light, e.g., LED (Light Emitting Diode) or laser light, needs to be illuminated for automatic focusing.
Meanwhile, with the supplementary light for automatic focusing, it is customary that the wavelength of the output light shall be on the low frequency side of the visible light range. Thus, if such supplementary light is used for automatic focusing for detecting the focusing position, the supplementary light undergoes chromatic aberration with respect to the lens, which differs from that of the light reflected from the object due to the difference in the frequency response between the supplementary light and the reflected light and therefore, it is difficult to effect correct control of the focusing position.
If there is no extraneous light other than a source of the supplementary light, the focusing position can be corrected by simply adding or subtracting the offset corresponding to the chromatic aberration to or from the lens movement position. Such correction may be effective if the imaging environment is completely dark. However, under an environment where there is the extraneous light in addition to the supplementary light, it is difficult to control the focusing position correctly, even on addition of the offset, because of difference in the conditions.